Influenza is a common infectious disease of the respiratory system associated with the Orthomyxoviridae family of viruses. Because of the high degree of variability of the virus, vaccination is typically required on a yearly basis with a reformulated vaccine that takes into account strain variations. The composition of the vaccine developed each year in the United States is determined by the Department of Food and Drug Administration Vaccines and the Related Biologicals Advisory Committee. The World Health Organization (WHO) similarly operates a global surveillance network of laboratories, for detection of new influenza variants, e.g., see Lavanchy, Vaccine 17:S24 (1999). Selection is based on antigenic analysis of recently isolated influenza viruses, the patterns of spread of antigenic variants, and the antibody responses of recently vaccinated subjects.
Influenza A and B are the two types of influenza viruses that cause epidemic human disease. Influenza A viruses are further categorized into subtypes on the basis of two surface antigens: hemagglutinin (HA) and neuraminidase (N). For example, the H1N1 subtype of influenza A viruses have a hemagglutinin type 1 antigen (H1) and a neuraminidase type 1 antigen (N1) while the H3N2 subtype have a hemagglutinin type 3 antigen (H3) and a neuraminidase type 2 antigen (N2). Influenza B viruses are not categorized into subtypes. Since 1977, influenza A (H1N1) viruses, influenza A (H3N2) viruses and influenza B viruses have been in global circulation. Vaccination is recognized as the single most effective way of preventing or attenuating influenza for those at high risk of serious illness from influenza infection and related complications. The inoculation of antigen prepared from inactivated influenza virus stimulates the production of specific antibodies. Protection is afforded only against those strains of virus from which the vaccine is prepared or closely related strains.
Each year's vaccine contains antigens from three virus strains (referred to as trivalent vaccine usually containing antigens from two type A strains and one type B strain) representing the influenza viruses that are believed likely to circulate in the coming winter. The antigenic characteristics of current and emerging influenza virus strains provide the basis for selecting strains included in each year's vaccine. The WHO reviews the world epidemiological situation annually and if necessary recommends new strains based on the current epidemiological evidence.
While influenza vaccines have been successful in reducing the incidence of influenza worldwide, there remains a need in the art for improved influenza vaccines that are stable and retain potency.